2,3,3,3-Tetrafluoropropene/vinylidene fluoride copolymers (poly-1234yf/VDF) have a variety of potential uses and applications. Depending on the two monomer unit ratios, poly-1234yf/VDF can be used as thermal plastics, coatings, elastomers, and membrane materials.
As membrane materials, one important application of fluoropolymers lies in the area of selective gas separations. The commercial examples in this field include TEFLON AF 2400 and AF 1600, HYFLON AD 80 and AD 60, and CYTOP. See Ind. Eng. Chem. Res. 2009, 48, 4638-4663. Recently, poly-1234yf/VDF has demonstrated superior separation properties for several gas pairs. International Publication No. WO 2012/112840 demonstrates good selectivities for O2/N2 and CO2/N2 separations using polymeric membranes made of 2,3,3,3-tetrafluoropropene/vinylidene fluoride copolymers having VDF as the major monomer unit. The high molecular weight 2,3,3,3-tetrafluoropropene/vinylidene fluoride copolymers with 2,3,3,3-tetrafluoropropene as the major monomer unit (about 90/10 mol %) exhibit excellent selectivities in CO2/CH4 and H2/CH4 separations, as disclosed in U.S. application Ser. No. 13/679,251.
As described in International Publication No. WO 2010/101304, 2,3,3,3-tetrafluoropropene/vinylidene fluoride copolymers can be synthesized by solution polymerization methods, i.e., at low temperatures (e.g., 25° C.) with a wide range of monomer unit ratios. However, the obtained copolymers with 2,3,3,3-tetrafluoropropene as the major monomer unit have relatively low molecular weight and low glass transition temperature.
2,3,3,3-Tetrafluoropropene/vinylidene fluoride copolymers can also be synthesized by an aqueous emulsion polymerization method. U.S. Patent Application Publication No. 2011/0097529 discloses the synthesis of 2,3,3,3-tetrafluoropropene/vinylidene fluoride copolymers by emulsion polymerization process at 83° C. or 110° C. International Publication Nos. WO 2012/125786 and WO 2012/125788 disclose the synthesis of 2,3,3,3-tetrafluoropropene/vinylidene fluoride copolymers using non-fluorinated surfactants in an aqueous emulsion process at 83° C. However, these patent publications primarily concern the synthesis of 2,3,3,3-tetrafluoropropene/vinylidene fluoride copolymers having VDF as the major monomer unit at relatively high temperatures.
In order to obtain high molecular weight polymers, polymerization is usually carried out at relatively low temperatures using redox pair initiators. However, at relatively low temperatures, for example 35 to 45° C., the initiation and propagation process of the aqueous emulsion copolymerization of 2,3,3,3-tetrafluoropropene/vinylidene fluoride with 2,3,3,3-tetrafluoropropene as the major monomer unit are very difficult, due to the relatively low polymerization reactivity of 2,3,3,3-tetrafluoropropene and the much lower boiling point of VDF which tends to be present in the vapor phase. Under these conditions, the copolymerization of 2,3,3,3-tetrafluoropropene/vinylidene fluoride with 2,3,3,3-tetrafluoropropene as the major monomer unit becomes sluggish with the initiation and propagation process depending on the reactor configuration and the stir rate. In addition, at low temperatures, the yields of the copolymerization are relatively low, though the relatively high amounts of redox pair initiators are used, which could result in, to some extent, the homopolymerization of 2,3,3,3-tetrafluoropropene.
There remains a need for improved synthesis of 2,3,3,3-tetrafluoropropene/vinylidene fluoride copolymers having 2,3,3,3-tetrafluoropropene as the major monomer unit.